A1 adenosine receptor (AR) agonists display antiischemic and antiepileptic neuroprotective activity, but peripheral cardiovascular side effects impeded their development. SAR study of N6-cycloalkylmethyl 4′-truncated (N)-methanocarba-adenosines identified 10 (MRS5474, N6-dicyclopropylmethyl, Ki 47.9 nM) as a moderately A1AR-selective full agonist. Two stereochemically defined N6-methynyl group substituents displayed narrow SAR; larger than cyclobutyl greatly reduced AR affinity, and larger or smaller than cyclopropyl reduced A1AR selectivity. Nucleoside docking to A1AR homology model characterized distinct hydrophobic cyclopropyl subpockets, the larger “A” forming contacts with Thr270 (7.35), Tyr271 (7.36), Ile274 (7.39) and carbon chains of glutamates (EL2), and smaller subpocket “B” between TM6 and TM7. 10 suppressed minimal clonic seizures (6 Hz mouse model) without typical rotarod impairment of A1AR agonists. Truncated nucleosides, an appealing preclinical approach, have more drug-like physicochemical properties than other A1AR agonists. Thus, we identified highly restricted regions for substitution around N6 suitable for an A1AR agonist with anticonvulsant activity.
G protein-coupled receptor; purines; molecular modeling; seizures; in vivo
(N)-Methanocarba adenosine 5′-methyluronamides containing known A3 AR (adenosine receptor)-enhancing modifications, i.e. 2-(arylethynyl)adenine and N6-methyl or N6-(3-substituted-benzyl), were nanomolar full agonists of human (h) A3AR and highly selective (Ki ~0.6 nM, N6-methyl 2-(halophenylethynyl) analogues 13, 14). Combined 2-arylethynyl-N6-3-chlorobenzyl substitutions preserved A3AR affinity/selectivity in the (N)-methanocarba series (e.g. 3,4-difluoro full agonist MRS5698 31, Ki 3 nM, human and mouse A3) better than for ribosides. Polyaromatic 2-ethynyl N6-3-chlorobenzyl analogues, such as potent linearly extended 2-p-biphenylethynyl MRS5679 34 (Ki hA3 3.1 nM; A1, A2A: inactive) and fluorescent 1-pyrene adduct MRS5704 35 (Ki hA3 68.3 nM) were conformationally rigid; receptor docking identified a large, mainly hydrophobic binding region. The vicinity of receptor-bound C2 groups was probed by homology modeling based on recent X-ray structure of an agonist-bound A2AAR, with a predicted helical rearrangement requiring an agonist-specific outward displacement of TM2 resembling opsin. Thus, X-ray structure of related A2AAR is useful in guiding design of new A3AR agonists.
G protein-coupled receptor; purines; molecular modeling; structure activity relationship; radioligand binding; adenylate cyclase
Novel D- and L-4′-thioadenosine derivatives lacking the 4′-hydroxymethyl moiety were synthesized, starting from D-mannose and D-gulonic γ-lactone, respectively, as potent and selective species-independent A3 adenosine receptor (AR) antagonists. Among the novel 4′-truncated 2-H nucleosides tested, a N6-(3-chlorobenzyl) derivative 7c was the most potent at the human A3 AR (Ki = 1.5 nM), but a N6-(3-bromobenzyl) derivative 7d showed the optimal species-independent binding affinity.
The structure activity relationship (SAR) of 1,2,4-triazolo[1,5-a]-1,3,5-triazine derivatives related to ZM241385 as antagonists of the A2A adenosine receptor (AR) was explored through the synthesis of analogues substituted at the 5 position. The A2A AR X-ray structure was used to propose a structural basis for the activity and selectivity of the analogues and to direct the synthetic design strategy to provide access to solvent-exposed regions. Thus, we have identified a point of substitution for the attachment of solubilizing groups to enhance both aqueous solubility and physicochemical properties, maintaining potent interactions with the A2A AR and, in some cases, receptor subtype selectivity. Among the most potent and selective novel compounds were a long-chain ether-containing amine congener 20 (Ki 11.5 nM) and its urethane-protected derivative 14 (Ki 17.8 nM). Compounds 20 and 31 (Ki 11.5 and 16.9 nM, respectively) were readily water soluble up to 10 mM. The analogues were docked in the crystallographic structure of the hA2A AR and in a homology model of the hA3 AR, and the per residue electrostatic and hydrophobic contributions to the binding were assessed and stabilizing factors were proposed.
G protein-coupled receptor; purines; molecular modeling; structure activity relationship; radioligand binding; adenylyl cyclase
Molecular modeling of agonist binding to the human A2A adenosine receptor (AR) was assessed and extended in light of crystallographic structures. Heterocyclic adenine nitrogens of co-crystallized agonist overlayed corresponding positions of the heterocyclic base of a bound triazolotriazine antagonist, and ribose moiety was coordinated in a hydrophilic region, as previously predicted based on modeling using the inactive receptor. Automatic agonist docking of 20 known potent nucleoside agonists to agonist-bound A2AAR crystallographic structures predicted new stabilizing protein interactions, to provide a structural basis for previous empirical structure activity relationships consistent with previous mutagenesis results. We predicted binding of novel C2 terminal amino acid conjugates of A2AAR agonist CGS21680 and used these models to interpret effects on binding affinity of newly-synthesized agonists. D-Amino acid conjugates were generally more potent than L- stereoisomers, and free terminal carboxylates more potent than corresponding methyl esters. Amino acid moieties were coordinated close to extracellular loops 2 and 3. Thus, molecular modeling is useful in probing ligand recognition and rational design of GPCR–targeting compounds with specific pharmacological profiles.
G protein-coupled receptor; nucleosides; purines; radioligand binding; docking; X-ray crystallography
Truncated N6-substituted-4′-oxo- and 4′-thioadenosine derivatives with C2 or C8 substitution were studied as dual acting A2A and A3 adenosine receptor (AR) ligands. The lithiation-mediated stannyl transfer and palladium-catalyzed cross coupling reactions were utilized for functionalization of the C2 position of 6-chloropurine nucleosides. An unsubstituted 6-amino group and a hydrophobic C2 substituent were required for high affinity at the hA2AAR, but hydrophobic C8 substitution abolished binding at the hA2AAR. However, most of synthesized compounds displayed medium to high binding affinity at the hA3AR, regardless of C2 or C8 substitution, and low efficacy in a functional cAMP assay. Several compounds tended to be full hA2AAR agonists. C2 substitution probed geometrically through hA2AAR-docking, was important for binding in order of hexynyl > hexenyl > hexanyl. Compound 4g was the most potent ligand acting dually as hA2AAR agonist and hA3AR antagonist, which might be useful for treatment of asthma or other inflammatory diseases.
lithiation-mediated stannyl transfer; structure-activity relationship; adenosine receptors; truncated adenosine; palladium-catalyzed cross coupling; dual-acting ligands
Sulfur-containing analogues of 8-substituted xanthines were prepared in an effort to increase selectivity or potency as antagonists at adenosine receptors. Either cyclopentyl or various aryl substituents were utilized at the 8-position, because of the association of these groups with high potency at A1-adenosine receptors. Sulfur was incorporated on the purine ring at positions 2 and/or 6, in the 8-position substituent in the form of 2- or 3-thienyl groups, or via thienyl groups separated from an 8-aryl substituent through an amide-containing chain. The feasibility of using the thienyl group as a prosthetic group for selective iodination via its Hg2+ derivative was explored. Receptor selectivity was determined in binding assays using membrane homogenates from rat cortex [[3H]-N6-(phenylisopropyl) adenosine as radioligand] or striatum [[3H]-5′-(N-ethylcarbamoyl)adenosine as radioligand] for A1- and A2-adenosine receptors, respectively. Generally, 2-thio-8-cycloalkylxanthines were at least as A1 selective as the corresponding oxygen analogue. 2-Thio-8-aryl derivatives tended to be more potent at A2 receptors than the oxygen analogue. 8-[4-[(Carboxymethyl)oxy]phenyl]-1,3-dipropyl-2-thioxanthine ethyl ester was >740-fold A1 selective.
The dedifferentiation agent ‘reversine’ (2-(4-morpholinoanilino)-N6-cyclohexyladenine 2) was found to be a moderately potent antagonist for the human A3 adenosine receptor (AR) with a Ki value 0.66 μM. This result prompted an exploration of the structure-activity relationship of related derivatives, synthesized via sequential substitution of 6-chloro-2-fluoropurine with selected nucleophiles. Optimization of substituents at these two positions identified 2-phenylamino-N6-(cyclohexyl)adenine 12, 2-phenylamino-N6-(cycloheptyl)adenine 19, and 2-phenylamino-N6-(endo-norbornyl)adenine 21 as potent A3 AR ligands with Ki values of 51, 42 and 37 nM, respectively, with 30 – 200-fold selectivity in comparison to A1 and A2A ARs. The most selective A3 AR antagonist (>200-fold) was 2-phenyloxy-N6-(cyclohexyl)adenine 22. 9-Methylation of 12, but not 19, was well tolerated in A3 AR binding. Extension of the 2-phenylamino group to 2-benzyl- and 2-(2-phenylethylamino) reduced affinity. In the series of 2-phenylamino, 2-phenyloxy, and 2-phenylthio substitutions, the order of affinity at the A3 AR was oxy ≥ amino > thio. Selected derivatives, including reversine (KB value of 466 nM in Schild analysis), competitively antagonized the functional effects of a selective A3 AR agonist, i.e. inhibition of forskolin-stimulated cAMP production in stably transfected Chinese hamster ovary (CHO) cells. These results are in agreement with other studies suggesting the presence of a lipophilic pocket in the AR binding site that is filled by moderately sized cycloalkyl rings at the N6 position of both adenine and adenosine derivatives. Thus, the compound series reported herein comprise an important new series of selective A3 AR antagonists. We were unable to reproduce the dedifferentiation effect of reversine, previously reported, or to demonstrate any connection between A3 AR antagonist effects and dedifferentiation.
The structure-activity relationships of 6-phenyl-1,4-dihydropyridine derivatives as selective antagonists at human A3 adenosine receptors have been explored (Jiang et al. J. Med. Chem.
1997, 39, 4667-4675). In the present study, related pyridine derivatives have been synthesized and tested for affinity at adenosine receptors in radioligand binding assays. Ki values in the nanomolar range were observed for certain 3,5-diacyl-2,4-dialkyl-6-phenylpyridine derivatives in displacement of [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)-5′-N-methylcarbamoyladenosine) at recombinant human A3 adenosine receptors. Selectivity for A3 adenosine receptors was determined vs radioligand binding at rat brain A1 and A2A receptors. Structure–activity relationships at various positions of the pyridine ring (the 3- and 5-acyl substituents and the 2- and 4-alkyl substituents) were probed. A 4-phenylethynyl group did not enhance A3 selectivity of pyridine derivatives, as it did for the 4-substituted dihydropyridines. At the 2-and 4-positions ethyl was favored over methyl. Also, unlike the dihydropyridines, a thioester group at the 3-position was favored over an ester for affinity at A3 adenosine receptors, and a 5-position benzyl ester decreased affinity. Small cycloalkyl groups at the 6-position of 4-phenylethynyl-1,4-dihydropyridines were favorable for high affinity at human A3 adenosine receptors, while in the pyridine series a 6-cyclopentyl group decreased affinity. 5-Ethyl 2,4-diethyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate, 38, was highly potent at human A3 receptors, with a Ki value of 20 nM. A 4-propyl derivative, 39b, was selective and highly potent at both human and rat A3 receptors, with Ki values of 18.9 and 113 nM, respectively. A 6-(3-chlorophenyl) derivative, 44, displayed a Ki value of 7.94 nM at human A3 receptors and selectivity of 5200-fold. Molecular modeling, based on the steric and electrostatic alignment (SEAL) method, defined common pharmacophore elements for pyridine and dihydropyridine structures, e.g., the two ester groups and the 6-phenyl group. Moreover, a relationship between affinity and hydrophobicity was found for the pyridines.
An alternative approach to overcome the inherent lack of specificity of conventional agonist therapy can be the reengineering of the GPCRs and their agonists. A reengineered receptor (neoceptor) could be selectively activated by a modified agonist, but not by the endogenous agonist. Assisted by rhodopsin-based molecular modeling, we pinpointed mutations of the A3 adenosine receptor (AR) for selective affinity enhancement following complementary modifications of adenosine. Ribose modifications examined included, at 3′: amino, aminomethyl, azido, guanidino, ureido; and at 5′: uronamido, azidodeoxy. N6-variations included: 3-iodobenzyl, 5-chloro-2-methyloxybenzyl, and methyl. An N6-3-iodobenzyl-3′-ureido adenosine derivative 10 activated phospholipase C in COS-7 cells (EC50=0.18 μM) or phospholipase D in chick primary cardiomyocytes mediated by a mutant (H272E), but not the wild-type, A3AR. The affinity enhancements for 10 and the corresponding 3′-acetamidomethyl analogue 6 were >100-fold and >20-fold, respectively. 10 concentration-dependently protected cardiomyocytes transfected with the neoceptor against hypoxia. Unlike 10, adenosine activated the wild-type A3AR (EC50 of 1.0 μM), but had no effect on the H272E mutant A3AR (100 μM). Compound 10 was inactive at human A1, A2A, and A2BARs. The orthogonal pair comprising an engineered receptor and a modified agonist should be useful for elucidating signaling pathways and could be therapeutically applied to diseases following organ-targeted delivery of the neoceptor gene.
Adenosine receptor agonists have cardioprotective, cerebroprotective, and antiinflammatory properties. We report that a carbocyclic modification of the ribose moiety incorporating ring constraints is a general approach for the design of A1 and A3 receptor agonists having favorable pharmacodynamic properties. While simple carbocyclic substitution of adenosine agonists greatly diminishes potency, methanocarba-adenosine analogues have now defined the role of sugar puckering in stabilizing the active adenosine receptor-bound conformation and thereby have allowed identification of a favored isomer. In such analogues a fused cyclopropane moiety constrains the pseudosugar ring of the nucleoside to either a Northern (N) or Southern (S) conformation, as defined in the pseudorotational cycle. In binding assays at A1, A2A, and A3 receptors, (N)-methanocarba-adenosine was of higher affinity than the (S)-analogue, particularly at the human A3 receptor (N/S affinity ratio of 150). (N)-Methanocarba analogues of various N6-substituted adenosine derivatives, including cyclopentyl and 3-iodobenzyl, in which the parent compounds are potent agonists at either A1 or A3 receptors, respectively, were synthesized. The N6-cyclopentyl derivatives were A1 receptor-selective and maintained high efficacy at recombinant human but not rat brain A1 receptors, as indicated by stimulation of binding of [35S]GTP-γ-S. The (N)-methanocarba-N6-(3-iodobenzyl)adenosine and its 2-chloro derivative had Ki values of 4.1 and 2.2 nM at A3 receptors, respectively, and were highly selective partial agonists. Partial agonism combined with high functional potency at A3 receptors (EC50 < 1 nM) may produce tissue selectivity. In conclusion, as for P2Y1 receptors, at least three adenosine receptors favor the ribose (N)-conformation.
1,3-Dialkylxanthine analogues containing carboxylic acid and other charged groups on 8-position substituents were synthesized. These derivatives were examined for affinity in radioligand binding assays at rat brain A3 adenosine receptors stably expressed in CHO cells using the new radioligand [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)adenosine-5′-N-methyluronamide), and at rat brain A1 and A2a receptors using [3H]PIA and [3H]CGS 21680, respectively. A synthetic strategy for introducing multiple carboxylate groups at the 8-position using iminodiacetic acid derivatives was explored. The presence of a sulfonate, a carboxylate, or multiple carboxylate groups did not result in a significant enhancement of affinity at rat A3 receptors, although as previously observed an anionic group tended to diminish potency at A1 and A2a receptors. The rat A3 receptor affinity was not highly dependent on the distance of a carboxylate group from the xanthine pharmacophore. 2-Thio vs 2-oxo substitution favored A3 potency, and 8-alkyl vs 8-aryl substitution favored A3 selectivity, although few derivatives were truly selective for rat A3 receptors. 1,3-Dimethyl-8-(3-carboxypropyl)-2-thioxanthine was 7-fold selective for A3 vs A2a receptors. 1,3,7-Trimethyl-8-(trans-2-carboxyvinyl)xanthine was somewhat selective for A3 vs A1 receptors. For 8-arylxanthines affinity at A3 receptors was enhanced by 1,3-dialkyl substituents, in the order dibutyl > dipropyl > diallyl.
The molecular basis for recognition by human P2Y1 receptors of the novel, competitive antagonist 2′-deoxy-N6-methyladenosine 3′,5′-bisphosphate (MRS 2179) was probed using site-directed mutagenesis and molecular modeling. The potency of this antagonist was measured in mutant receptors in which key residues in the transmembrane helical domains (TMs) 3, 5, 6, and 7 were replaced by Ala or other amino acids. The capacity of MRS 2179 to block stimulation of phospholipase C promoted by 2-methylthioadenosine 5′-diphosphate (2-MeSADP) was lost in P2Y1 receptors having F226A, K280A, or Q307A mutations, indicating that these residues are critical for the binding of the antagonist molecule. Mutation of the residues His132, Thr222, and Tyr136 had an intermediate effect on the capacity of MRS 2179 to block the P2Y1 receptor. These positions therefore appear to have a modulatory role in recognition of this antagonist. F131A, H277A, T221A, R310K, or S317A mutant receptors exhibited an apparent affinity for MRS 2179 that was similar to that observed with the wild-type receptor. Thus, Phe131, Thr221, His277, and Ser317 are not essential for antagonist recognition. A computer-generated model of the human P2Y1 receptor was built and analyzed to help interpret these results. The model was derived through primary sequence comparison, secondary structure prediction, and three-dimensional homology building, using rhodopsin as a template, and was consistent with data obtained from mutagenesis studies. We have introduced a “cross-docking” procedure to obtain energetically refined 3D structures of the ligand–receptor complexes. Cross-docking simulates the reorganization of the native receptor structure induced by a ligand. A putative nucleotide binding site was localized and used to predict which residues are likely to be in proximity to agonists and antagonists. According to our model TM6 and TM7 are close to the adenine ring, TM3 and TM6 are close to the ribose moiety, and TM3, TM6, and TM7 are near the triphosphate chain.
The M1-selective muscarinic receptor antagonist pirenzepine (5,11-dihydro-11-[(4-methyl-1-piperazinyl)acetyl]-6H-pyrido[2,3-b] [1,4]benzodiazepin-6-one) was derivatized to explore points of attachment of functionalized side chains for the synthesis of receptor probes and ligands for affinity chromatography. The analogues prepared were evaluated in competitive binding assays versus [3H]-N-methylscopolamine at four muscarinic receptor subtypes (m1AChR-m4AChR) in membranes from rat heart tissue and transfected A9L cells. 9-(Hydroxymethyl)pirenzepine, 8-(methylthio)pirenzepine, and a series of 8-aminosulfonyl derivatives were synthesized. Several 5-substituted analogues of pirenzepine also were prepared. An alternate series of analogues substituted on the 4-position of the piperazine ring was prepared by reaction of 4-desmethylpirenzepine with various electrophiles. An N-chloroethyl analogue of pirenzepine was shown to form a reactive aziridine species in aqueous buffer yet failed to affinity label muscarinic receptors. Within a series of aminoalkyl analogues, the affinity increased as the length of the alkyl chain increased. Shorter chain analogues were generally much less potent than pirenzepine, and longer analogues (7–10 carbons) were roughly as potent as pirenzepine at m1 receptors, but were nonselective. Depending on the methylene chain length, acylation or alkyl substitution of the terminal amine also influenced the affinity at muscarinic receptors.
A series of functionalized congeners of adenosine based on N6-phenyladenosine, a potent A1-adenosine receptor agonist, was synthesized. Derivatives of the various congeners should be useful as receptor and histochemical probes and for the preparation of radioligands and affinity columns or as targeted drugs. N6-[4-(Carboxymethyl)phenyl]adenosine served as the starting point for synthesis of the methyl ester, the methyl amide, the ethyl glycinate, and various substituted anilides. One of the latter, N6-[4-[[[4-(carbomethoxymethyl)anilino]carbonyl]methyl]phenyl]adenosine, served as the starting point for the synthesis of another series of congeners including the methyl amide, the hydrazide, and the aminoethyl amide. The terminal amino function of the last congener was acylated to provide further analogues. The various congeners were potent competitive antagonists of binding of N6-[3H]cyclohexyladenosine to A1-adenosine receptors in rat cerebral cortical membranes. The affinity of the congener for the A1 receptor was highly dependent on the nature of the spacer group and the terminal moiety with Ki values ranging 1–100 nM. A biotinylated analogue had a Ki value of 11 nM. A conjugate derived from the Bolton–Hunter reagent had a Ki value of 4.5 nM. The most potent congener contained a terminal [(aminoethyl)amino]carbonyl function and had a Ki value of less than 1 nM.
A series of functionalized congeners of 1,3-dialkylxanthines has been prepared as adenosine receptor antagonists. On the basis of the high potency of 8-(p-hydroxyphenyl)-1,3-dialkylxanthines, the parent compounds were 8-[4-[(carboxymethyl)oxy]phenyl] derivatives of theophylline and 1,3-dipropylxanthine. A series of analogues including esters of ethanol and N-hydroxysuccinimide, amides, a hydrazide, an acylurea, and anilides were prepared. The potency in blocking A1-adenosine receptors (inhibition of binding of N6-[3H]cyclohexyladenosine to brain membranes) and A2-adenosine receptors (inhibition of 2-chloroadenosine-elicited accumulations of cyclic AMP in brain slices) was markedly affected by structural changes distal to the primary pharmacophore (8-phenyl-1,3-dialkylxanthine). Potencies in the dipropyl series at the A1 receptor ranged from K1 values of 1.2 nM for a congener with a terminal amidoethyleneamine moiety to a K1 value of 58 nM for the parent carboxylic acid to a K1 of 96 nM for the bulky ureido congener. Certain congeners were up to 145-fold more active at A1 receptors than at A2 receptors. Various derivatives of the congeners should be useful as receptor probes and for radioidodination, avidin binding, and preparation of affinity columns.
Adenosine derivatives bearing an N6-(3-iodobenzyl) group, reported to enhance the affinity of adenosine-5′-uronamide analogues as agonists at A3 adenosine receptors (J. Med. Chem.
37, 636–646), were synthesized starting from methyl β-d-ribofuranoside in 10 steps. Binding affinities at A1 and A2a receptors in rat brain membranes and at cloned rat A3 receptors from stably transfected CHO cells were compared. N6-(3-Iodobenzyl)adenosine was 2-fold selective for A3 vs A1 or A2a receptors; thus it is the first monosubstituted adenosine analogue having any A3 selectivity. The effects of 2-substitution in combination with modifications at the N6- and 5′-positions were explored. 2-Chloro-N6-(3-iodobenzyl)adenosine had a Ki value of 1.4 nM and moderate selectivity for A3 receptors. 2-Chloro-N6-(3-iodobenzyl)adenosine-5′-N-methyluronamide, which displayed a Ki value of 0.33 nM, was selective for A3 vs A1 and A2a receptors by 2500- and 1400-fold, respectively. It was 46,000-fold selective for A3 receptors vs the Na+-independent adenosine transporter, as indicated in displacement of [3H]N6-(4-nitrobenzyl)-thioinosine binding in rat brain membranes. In a functional assay in CHO cells, it inhibited adenylate cyclase via rat A3 receptors with an IC50 of 67 nM. 2-(Methylthio)-N6-(3-iodobenzyl)-adenosine-5′-N-methyluronamide and 2-(methylamino)-N6-(3-iodobenzyl)adenosine-5′-N-methyluronamide were less potent, but nearly as selective for A3 receptors. Thus, 2-substitution (both small and sterically bulky) is well-tolerated at A3 receptors, and its A3 affinity-enhancing effects are additive with effects of uronamides at the 5′-position and a 3-iodobenzyl group at the N6-position.
A series of ring-constrained (N)-methanocarba-5′-uronamide 2,N6-disubstituted adenine nucleosides have been synthesized via Mitsunobu condensation of the nucleobase precursor with a pseudosugar ring containing a 5′-ester functionality. Following appropriate functionalization of the adenine ring, the ester group was converted to the 5′-N-methylamide. The compounds, mainly 2-chloro substituted derivatives, were tested in both binding and functional assays at human adenosine receptors (ARs), and many were found to be highly potent and selective A3AR agonists. Selected compounds were compared in binding to the rat A3AR to assess their viability for testing in rat disease models. The N6-(3-chlorobenzyl) and N6-(3-bromobenzyl) analogues displayed Ki values at the human A3AR of 0.29 and 0.38 nM, respectively. Other subnanomolar affinities were observed for the following N6 derivatives: 2,5-dichlorobenzyl, 5-iodo-2-methoxybenzyl, trans-2-phenyl-1-cyclopropyl, and 2,2-diphenylethyl. Selectivity for the human A3AR in comparison to the A1AR was (fold): the N6-(2,2-diphenylethyl) analogue 34 (1900), the N6-(2,5-dimethoxybenzyl) analogue 26 (1200), the N6-(2,5-dichlorobenzyl) and N6-(2-phenyl-1-cyclopropyl) analogues 20 and 33 (1000), and the N6-(3-substituted benzyl) analogues 17, 18, 28, and 29 (700–900). Typically, even greater selectivity ratios were obtained in comparison with the A2A and A2BARs. The (N)-methanocarba-5′-uronamide analogues were full agonists at the A3AR, as indicated by the inhibition of forskolin-stimluated adenylate cyclase at a concentration of 10 µM. The N6-(2,2-diphenylethyl) derivative was an A3AR agonist in the (N)-methanocarba-5′-uronamide series, although it was an antagonist in the ribose series. Thus, many of the previously known groups that enhance A3AR affinity in the 9-riboside series, including those that reducing intrinsic efficacy, may be adapted to the (N)-methanocarba nucleoside series of full agonists.
The binding affinities at rat A1, A2a, and A3 adenosine receptors of a wide range of derivatives of adenosine have been determined. Sites of modification include the purine moiety (1-, 3-, and 7-deaza; halo, alkyne, and amino substitutions at the 2- and 8-positions; and N6-CH2-ring, -hydrazino, and -hydroxylamino) and the ribose moiety (2′-, 3′-, and 5′-deoxy; 2′- and 3′-O-methyl; 2′-deoxy 2′-fluoro; 6′-thio; 5′-uronamide; carbocyclic; 4′- or 3′-methyl; and inversion of configuration). (−)- and (+)-5′-Noraristeromycin were 48- and 21-fold selective, respectively, for A2a vs A1 receptors. 2-Chloro-6′-thioadenosine displayed a Ki value of 20 nM at A2a receptors (15-fold selective vs A1). 2-Chloroadenin-9-yl(β-L-2′-deoxy-6′-thiolyxofuranoside) displayed a Ki value of 8 μM at A1 receptors and appeared to be an antagonist, on the basis of the absence of a GTP-induced shift in binding vs a radiolabeled antagonist (8-cyclopentyl-1,3-dipropylxanthine). 2-Chloro-2′-deoxyadenosine and 2-chloroadenin-9-yl(β-D-6′-thioarabinoside) were putative partial agonists at A1 receptors, with Ki values of 7.4 and 5.4 μM, respectively. The A2a selective agonist 2-(1-hexynyl)-5′-(N-ethylcarbamoyl)adenosine displayed a Ki value of 26 nM at A3 receptors. The 4′-methyl substitution of adenosine was poorly tolerated, yet when combined with other favorable modifications, potency was restored. Thus, N6-benzyl-4′-methyladenosine-5′-(N-methyluronamide) displayed a Ki value of 604 nM at A3 receptors and was 103- and 88-fold selective vs A1 and A2a receptors, respectively. This compound was a full agonist in the A3-mediated inhibition of adenylate cyclase in transfected CHO cells. The carbocyclic analogue of N6-(3-iodobenzyl)adenosine-5′-(N-methyluronamide) was 2-fold selective for A3 vs A1 receptors and was nearly inactive at A2a receptors.
Structure–affinity relationships for ligand binding at the human A2A adenosine receptor have been probed using site-directed mutagenesis in the transmembrane helical domains (TMs). The mutant receptors were expressed in COS-7 cells and characterized by binding of the radioligands [3H]CGS21680, [3H]NECA, and [3H]XAC. Three residues, at positions essential for ligand binding in other G protein-coupled receptors, were individually mutated. The residue V(3.32) in the A2A receptor that is homologous to the essential aspartate residue of TM3 in the biogenic amine receptors, i.e., V84(3.32), may be substituted with L (present in the A3 receptor) but not with D (in biogenic amine receptors) or A. H250(6.52), homologous to the critical N507 of rat m3 muscarinic acetylcholine receptors, may be substituted with other aromatic residues or with N but not with A (Kim et al. J. Biol. Chem.
1995, 270, 13987–13997). H278(7.43), homologous to the covalent ligand anchor site in rhodopsin, may not be substituted with either A, K, or N. Both V84L(3.32) and H250N(6.52) mutant receptors were highly variable in their effect on ligand competition depending on the structural class of the ligand. Adenosine-5′-uronamide derivatives were more potent at the H250N(6.52) mutant receptor than at wild type receptors. Xanthines tended to be close in potency (H250N(6.52)) or less potent (V84L-(3.32)) than at wild type receptors. The affinity of CGS21680 increased as the pH was lowered to 5.5 in both the wild type and H250N(6.52) mutant receptors. Thus, protonation of H250-(6.52) is not involved in this pH dependence. These data are consistent with a molecular model predicting the proximity of bound agonist ligands to TM3, TM5, TM6, and TM7.
9-Alkyladenine derivatives and ribose-modified N6-benzyladenosine derivatives were synthesized in an effort to identify selective ligands for the rat A3 adenosine receptor and leads for the development of antagonists. The derivatives contained structural features previously determined to be important for A3 selectivity in adenosine derivatives, such as an N6-(3-iodobenzyl) moiety, and were further substituted at the 2-position with halo, amino, or thio groups. Affinity was determined in radioligand binding assays at rat brain A3 receptors stably expressed in Chinese hamster ovary (CHO) cells, using [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)adenosine-5′-(N-methyluronamide)), and at rat brain A1 and A2a receptors using [3H]-N6-PIA ((R)-N6-phenylisopropyladenosine) and [3H]CGS 21680 (2-[[[4-(2-carboxyethyl)-phenyl]ethyl]amino]-5′-(N-ethylcarbamoyl)adenosine), respectively. A series of N6-(3-iodobenzyl) 2-amino derivatives indicated that a small 2-alkylamino group, e.g., methylamino, was favored at A3 receptors. N6-(3-Iodobenzyl)-9-methyl-2-(methylthio)adenine was 61-fold more potent than the corresponding 2-methoxy ether at A3 receptors and of comparable affinity at A1 and A2a receptors, resulting in a 3–6-fold selectivity for A3 receptors. A pair of chiral N6-(3-iodobenzyl) 9-(2,3-dihydroxypropyl) derivatives showed stereoselectivity, with the R-enantiomer favored at A3 receptors by 5.7-fold. 2-Chloro-9-(β-d-erythrofuranosyl)-N6-(3-iodobenzyl)adenine had a Ki value at A3 receptors of 0.28 µM. 2-Chloro-9-[2-amino-2,3-dideoxy-β-d-5-(methylcarbamoyl)-arabinofuranosyl]-N6-(3-iodobenzyl)adenine was moderately selective for A1 and A3 vs A2a receptors. A 3′-deoxy analogue of a highly A3-selective adenosine derivative retained selectivity in binding and was a full agonist in the inhibition of adenylyl cyclase mediated via cloned rat A3 receptors expressed in CHO cells. The 3′-OH and 4′-CH2OH groups of adenosine are not required for activation at A3 receptors. A number of 2′,3′-dideoxyadenosines and 9-acyclic-substituted adenines appear to inhibit adenylyl cyclase at the allosteric “P” site.
Functionalized congeners derived from 1,3-dipropyl-8-phenylxanthine and from N6-phenyladenosine were derivatized to contain electrophilic groups (isothiocyanate, N-hydroxysuccinimide ester, maleimide, sulfonyl chloride, or α-haloacyl group) capable of reaction with nucleophiles on biopolymers. The goal was to inhibit chemically the A1 adenosine receptor by using reactive agonist and antagonist ligands. Some of the electrophilic derivatives were synthesized through acylation of amine-functionalized congeners using hetero- or homobifunctional reagents available for protein cross-linking. The affinity for A1 adenosine receptors was evaluated in competitive binding assays by using rat and bovine brain membranes. Several xanthine and adenosine thiourea derivatives prepared from 1,3- and l,4-phenylene diisothiocyanate (DITC) were potent irreversible inhibitors of adenosine receptors. Derivatives of m-DITC, at concentrations between 10 and 500 nM, irreversibly eliminated binding at 90% of the A1-receptor sites. Receptor affinity of both xanthine and adenosine derivatives containing distal phenylthiourea substituents was diminished by electron-donating groups on the ring.
Amide derivatives of a carboxylic acid congener of 1,3-dialkylxanthine, having a 4-[(carboxymethyl)oxy]phenyl substituent at the 8-position, have been synthesized in order to identify potent antagonists at A2-adenosine receptors stimulatory to adenylate cyclase in platelets. Distal structural features of amide-linked chains and the size of the 1,3-dialkyl groups have been varied. 1,3-Diethyl groups, more than 1,3-dimethyl or 1,3-dipropyl groups, favor A2 potency, even in the presence of extended chains attached at the 8-(p-substituted-phenyl) position. Polar groups, such as amines, on the chain simultaneously enhance water solubility and A2 potency. Among the most potent A2 ligands are an amine congener, 8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]oxy]phenyl]-1,3-diethylxanthine, and its D-lysyl conjugate, which have KB values of 21 and 23 nM, respectively, for the antagonism of N-ethyl-adenosine-5′-uronamide-stimulated adenylate cyclase activity in human platelet membranes. Strategies for the selection and tritiation of new radioligands for use in competitive binding assays at A2-adenosine receptors have been considered.
Human P2Y receptors encompass at least eight subtypes of Class A G protein-coupled receptors (GPCRs), responding to adenine and/or uracil nucleotides. Using a BLAST search against the Homo sapiens subset of the SWISS–PROT and TrEMBL databases, we identified 68 proteins showing high similarity to P2Y receptors. To address the problem of low sequence identity between rhodopsin and the P2Y receptors, we performed a multiple-sequence alignment of the retrieved proteins and the template bovine rhodopsin, combining manual identification of the transmembrane domains (TMs) with automatic techniques. The resulting phylogenetic tree delineated two distinct subgroups of P2Y receptors: Gq-coupled subtypes (e.g., P2Y1) and those coupled to Gi (e.g., P2Y12). On the basis of sequence comparison we mutated three Tyr residues of the putative P2Y1 binding pocket to Ala and Phe and characterized pharmacologically the mutant receptors expressed in COS-7 cells. The mutation of Y306 (7.35, site of a cationic residue in P2Y12) or Y203 in the second extracellular loop selectively decreased the affinity of the agonist 2-MeSADP, and the Y306F mutation also reduced antagonist (MRS2179) affinity by 5-fold. The Y273A (6.48) mutation precluded the receptor activation without a major effect on the ligand-binding affinities, but the Y273F mutant receptor still activated G proteins with full agonist affinity. Thus, we have identified new recognition elements to further define the P2Y1 binding site and related these to other P2Y receptor subtypes. Following sequence-based secondary-structure prediction, we constructed complete models of all the human P2Y receptors by homology to rhodopsin. Ligand docking on P2Y1 and P2Y12 receptor models was guided by mutagenesis results, to identify the residues implicated in the binding process. Different sets of cationic residues in the two subgroups appeared to coordinate phosphate-bearing ligands. Within the P2Y1 subgroup these residues are R3.29, K/R6.55, and R7.39. Within the P2Y12 subgroup, the only residue in common with P2Y1 is R6.55, and the role of R3.29 in TM3 seems to be fulfilled by a Lys residue in EL2, whereas the R7.39 in TM7 seems to be substituted by K7.35. Thus, we have identified common and distinguishing features of P2Y receptor structure and have proposed modes of ligand binding for the two representative subtypes that already have well-developed ligands.